In modern mobile telecommunications systems a mobile station may have a choice as to whether to operate with a public cellular system or with an autonomous system, such ti as a residential system of a private system Typically it will be desirable to operate with a selected autonomous system, which may provide a more favorable rate structure than the public cellular system(s), or that may provide a desired service not offered by the public cellular system(s). A particular autonomous system may be a residential system that serves the user's home, or a private system that serves the user's workplace.
One such modern cellular system is referred to as IS-136, which is described in IS-136.1, Rev. A, February 1996, and subsequent updated releases. This system employs Digital Control Channels (DCCHs) that enable a mobile station to gain access to the system. When a mobile station scans for and subsequently monitors a DCCH, it is said to be "ucamped" on that particular DCCH. Page messages and other information are received from the DCCH.
In Section 6.3.19 of IS-136.1 there is described a non-public mode search procedure that is to be implemented by IS-136 compliant mobile stations. As defined, while camping on a DCCH a mobile station user may decide to initiate a Non-Public Mode Search Condition in order to search for service with an alternate system (System Identification (SID), Private System Identification (PSID), or Residential System Identification (RSID)) on the current DCCH and/or other DCCHs. Two possible procedures are defined for Non-Public Mode Search: New PSID/RSID Search and Manual System Search.
Discussing first the New PSID/RSID Search procedure, when the user invokes this procedure the mobile station proceeds to collect one signal strength measurement on each frequency in the current frequency band. Next, the mobile station makes a list of up to 24 channels with the strongest signals, and then tunes to the strongest channel in the list. The mobile station then determines if this channel contains a DCCH. If the channel contains a DCCH, the mobile station reads the Fast Broadcast Control Channel (F-BCCH) and determines therefrom if the DCCH is marked with a non-public Network Type (Private and/or Residential) that is enabled in the mobile station. If this is the case, the mobile station marks the DCCH as DCCH.sub.-- 1. If the channel does not contain a DCCH, or if the DCCH is not marked with a non-public Network Type that is enabled in the mobile station, then a determination is made if this is the last channel in the channel list. If it is, the procedure ends, otherwise the mobile station reads the next strongest channel in the channel list and the process repeats.
After possibly performing a Test Registration procedure, and marking an appropriate PSID/RSID as SYS.sub.-- 1, the mobile station displays an indication of SYS.sub.-- 1 to the user. This enables the user to select SYS.sub.-- 1, using the mobile station's user interface (e.g., keypad). After other processing, a determination is made if DCCH.sub.-- 1 is the current DCCH. If it is not, the mobile station adds DCCH.sub.-- 1 to a list of channels identified as requiring measurements (see Section 6.3.3.1, Control Channel Locking) The mobile station then, after an appropriate delay required for channel measurement purposes (see Section 6.3.3.3), declares a Priority System Condition (see Section 6.2.3) using DCCH.sub.-- 1 as the only reselection candidate. The CELLTYPE for DCCH.sub.-- 1 defaults to PREFERRED until otherwise determined In addition, the mobile station determines the MS.sub.-- ACC PWR, RSS.sub.-- ACC MIN, SS .sub.-- SUFF and DELAY (see Section 6.3.3.4.2) for DCCH.sub.-- 1 prior to involving or while executing the Control Channel Reselection procedure (see Section 6.3.3).
In the Manual System Search procedure, the mobile station proceeds as follows. The mobile station first searches the current DCCH and neighboring DCCHs (including Private Operating Frequencies (POFs) if they exist) for all candidates that support one or more of the PSIDs, RSIDs, and all SIDs that the mobile station subscribes to. The mobile station then displays a PSID/RSID Alphanumeric Name of each PSID or RSID supported by the candidate control channels that match a PSID or RSID stored in the mobile station, and the Alphanumeric System ID of the preferred SID. The mobile station then marks as DCCH.sub.-- 1 the candidate control channel supporting the SID, PSID, or RSID matching the Alphanumeric System ID or PSID/RSID Alphanumeric Name selected by the user using the mobile station's user interface. If more than one candidate control channel supports the selected PSID, RSID, or SID, then the candidate with the highest signal strength is marked as DCCH.sub.-- 1. If no SID, PSID, or RSID is selected by the user, the procedure is simply terminated.
If DCCH.sub.-- 1 is not the current DCCH, then the mobile station performs the same steps as described above for the New PSID/RSID search procedure, i.e., adding DCCH.sub.-- 1 to the list of channels identified as requiring measurements, etc.
Section 6.2.3 of IS-136.1 specifies the operation of the mobile station in a DCCH Camping State. Upon entering this state from a Control Channel Scanning and Locking state (Section 6.2.2), or for the first time as a result of control channel reselection, the mobile station always makes an initial reading of a full cycle of the F-BCCH and the Extended Broadcast Control Channel (E-BCCH). When operating in this state the mobile station does not make an access attempt until it has completed its initial reading of a full cycle of the F-BCCHI. After completing its initial reading of the F-BCCH, the mobile station leaves the DCCH Camping state in order to process any of a number of specified transactions These transactions include, but are not limited to, the occurrence of an originated or a terminated call, a registration, and an originated point to point teleservice.
The Layer 3 controller of the mobile station remains in the DCCH camping state as long as it is logically connected to the best DCCH according to control channel (re)selection rules. While in the DCCH Camping State the mobile station is specified to respond to the occurrence of a number of different conditions. These include, but are not limited to, a determination that an Analog Control Channel (ACC) is a preferred service provider, a radio link failure, and, of most interest to this invention, a Periodic Evaluation condition.
The Periodic Evaluation condition, as currently-defined, exists whenever the mobile station decides to evaluate the signal strength performance of other DCCHs. This condition is specified to occur at least once every SCANINTERVAL (see Section 6.3.3.1) when a Full.sub.-- Reselect.sub.-- Data.sub.-- Indicator is set (see Section 6.3.3.3). Whenever this condition is detected the mobile station invokes a Control Channel Reselection procedure (see Section 6.3.3) using the results obtained according to a Signal Strength Measurements procedure (see Section 6.3.2.3). A Neighbor List having entries acquired on the current DCCH, Private Operating Frequencies (see Section 6.3.21) and/or a DCCH identified as a result of a Non-Public Mode Search (see Section 6.3.19) can all serve as candidates for cell reselection.
However, this conventional technique causes a problem when the user has manually selected either a SID, PSID, or RSID using the procedure described above. In this case, and assuming that the mobile station has camped on the DCCH of the manually selected system, the above-described Periodic Evaluation procedure will result almost immediately in the mobile station beginning to scan for another, stronger DCCH. This can result in the mobile station reselecting to another DCCH, thereby defeating and overriding the user's manual selection of a particular system. It can be appreciated that this type of operation will be objectionable to the user.